This invention relates to superfatted soaps and more particularly to soaps which have been superfatted by the use of certain fatty acids whereby the lathering of the superfatted soap is enhanced and the tendency of the soap to become rancid is reduced. The superfatted soap is preferably of toilet quality in the form of bars.
Soaps employing superfatting agents of many kinds are known in the art and are commercially available. Such superfatting agents can include lanolin, higher fatty alcohols, mineral oils and higher fatty acids. The use of free fatty acids as a superfatting agent in soap is described in U.S. Pat. No. 3,576,749 to Megson. According to this patent, the presence of free fatty acids such as lauric, palmitic, stearic, oleic and others in the soap product improves the volume and quality of lather, causing it to be more stable with smaller air bubbles which give a lather characterized as richer and creamier, and is also alleged to tend to soften skin. Thus the incorporation of free fatty acids into a soap bar is desirable in that it helps eliminate free alkali, lowers the pH and may make the soap milder. It is also indicated to improve the lathering characteristics of the bar.
Our studies indicate that the customary free fatty acids added to soaps as a superfatting agent and the soap molecules reach an equilibrium distribution by migration. This migration tends to be in the direction of the free fatty acid portion having a composition similar to that of the total soap. For example, consider a soap base consisting of 70 parts sodium tallowate and 30 parts sodium cocoate to which has been added 10% by weight of coco fatty acids as a superfatting agent. The coco fatty acids normally contain about 50% of lauric acid (C.sub.12). After the soap was processed and allowed to stand for a period, analysis of the free fat portion of the soap showed that the lauric acid content had dropped from about 50% to about 17.6% which is about the lauric acid content of the entire soap product. Analysis of the free fat of this same soap base also showed that the unsaturated fatty acid portion (oleic acid) had increased from an initial 5% to about 42.3%. The tallow portion of such a 70/30 ratio tallow/coco base contains about 42% of oleic acid.
Thus, by migration, the free fat portion of a soap made with tallow fatty acids, when superfatted with a fatty acid such as coco, could contain a large amount of unsaturated fatty acids such as oleic, linoleic and linolenic, all of which are particularly prone to rancidity.
We have also found that with a soap base consisting of 90 parts by weight sodium laurate soap and 10 parts by weight oleic acid as the superfatting agent, an analysis of the superfatting agent after a period of time shows a mixture of free lauric and free oleic acid in about a 9:1 ratio. This shows that by virtue of migration, the ratio of free oleic and free lauric acid in the superfatting agent is about the same as in the original mixture, that is, 90 parts of sodium laurate with 10 parts of oleic acid. Thus, migration causes a distribution of fatty acid chains similar to that of the overall soap product even though the acid used to superfat the soap has a different distribution. This migration is about the same whether the free fatty acid is added to the finished soap pellets or to the neat soap.
This migration of fatty acids can be a problem. Since tallow fatty acids are a prime acid source for producing soaps and tallow fatty acid is composed mainly of oleic, linoleic and linolenic acid which are unsaturated acids, through migration there will be a greater proportion of oleic acid in the superfatting agent, and this can lead to rancidity problems.